Byleth has a very subdued personality and has difficulty expressing emotion. As a newborn, they neither laughed nor cried. People they are especially close to, such as Jeralt, are able to tell how they are feeling despite their limited emotional visualizations. A handful of their students note that their outward expressions hardly vary. Some express surprise whenever they simply smile.

I'm open to interpretation for this role but if you wish to know what her character actually sounds like:
(English version) Watch from 6:40
https://youtu.be/Z6qRL0aejNc?t=400
(English and Japanese versions) Maybe spoiler if you haven't reached Post-Time Skip
https://www.youtube.com/watch?v=NKt4HRFvslM

Pronunciation of Byleth's name - https://youtu.be/b0exSwY6XhM?t=56 00:56 and 1:10.

I will be using the auditions lines for the lines in the video so please keep that in mind for when you hand in your lines. Please feel free to give a min of two takes and a max of four takes per line with variations.

Please use the comic as you record so you can match her expressions 
https://moeblob.tumblr.com/post/183371699275